This invention pertains to a process for reacting isocyanates with substances containing isocyanate groups or active hydrogen groups such as hydroxyl, amino or like groups in the presence of a sulfonium zwitterion which initiates the reaction. This invention also relates to the products of the aforementioned reaction.
Reactions of isocyanates with active hydrogen compounds such as polyols to form polyurethanes are well known as described in Polyurethanes: Chemistry and Technology I. Chemistry, Saunders and Frisch, Interscience, 1962, pp. 63-128. Such reactions are commonly catalyzed by tertiary amines, organometallic compounds, phosphines, triazines and mixtures thereof. The polyurethanes are usually produced in the form of foams, elastomers, coatings, adhesives, fibers and films, and most often as flexible and rigid foams.
In the production of foams, the cellular polyurethane is made by reacting an isocyanate with polyol -- generally a polyfunctional hydroxyl-terminated polyether or polyester -- in a formulation also containing a blowing agent and a surfactant in addition to the reactants and catalyst. By proper choice of isocyanate and polyol, foams can be made with properties ranging from flexible to semi-rigid to rigid. As is well known, the polyurethane foams, particularly the rigid foams, are excellent thermal insulators in a wide variety of applications, especially in the refrigerant and construction industries.
Unfortunately, the flammability and smoke evolutionary character of polyurethane foams has significantly limited the use of such foams in many applications. Conventionally, the burning characteristics of polyurethane foams are reduced by adding halogenated organic materials or combinations thereof with phosphorus compounds to the polyurethane formulation. More recently, improved heat resistant polyurethanes have been made by introducing heat resistant groups such as isocyanurates (often called isocyanurate rings) into the polyurethane network. In addition to high thermal stability, these isocyanurate rings and the polyurethanes containing them also exhibit hydrolytic stability. Moreover, foamed polyurethanes containing the isocyanurate groups exhibit improved dimensional stability.
One of the best methods for introducing the isocyanurate groups into the polyurethane is to employ partially trimerized isocyanate or similar isocyanate-terminated oligomers (often called NCO-terminated prepolymers) as at least a part of the isocyanate component in the polyurethane formulation. Alternatively, the isocyanurate groups may be introduced simultaneously with the formation of the polyurethanes (so-called one-step method) by reacting isocyanate and polyol in one vessel in the presence of the catalyst. Such NCO-terminated oligomers are conventionally formed by contacting an isocyanate with a catalyst which may be an amine, phosphine, metal alkoxide, metal oxides, organometallics, metal chelates and combinations thereof. Unfortunately, relatively large amounts of catalyst are required to produce the desired trimerization, particularly in the preparation of isocyanurate foams. The presence of the larger amounts of catalysts, particularly the amine or metal catalysts, often cause degradation where the reaction product is subjected to elevated temperatures. Also, the amine catalysts are notorious for causing odor problems.
More importantly, partially trimerized isocyanates and other NCO-terminated oligomers prepared by using the aforementioned conventional catalysts are not sufficiently stable to withstand the conditions of lengthy storage and/or shipping. As a result such oligomers undergo significant additional polymerization (often to and beyond the point of gelation) before actual use. It is believed that such instability toward further molecular weight increase is caused by residues of catalyst remaining in the oligomers.
In the preparation of flexible polyurethane foams wherein foaming is partly caused by CO.sub.2 released from the reaction water with isocyanate groups, a dual catalyst is required in order to obtain a foam having acceptable physical properties and/or to facilitate processing. This dual catalyst commonly consists of a tertiary amine and an organometallic compound. Due to the instability of the organometallic compound, usually a tin alkanoate such as tin octoate, these dual catalysts have to be carefully metered in separate streams in commercial foam machines. Also, the amine compound causes undesirable odor problems.
In view of the aforementioned difficulties with the conventional catalysts employed in the reactions of isocyanates, it would be highly desirable to provide more stable, NCO-terminated isocyanurate oligomers (often called partially trimerized isocyanates) as well as an improved process for carrying out isocyanate reactions in general.